Go, Go, Karate Rangers!

Martial artists are always demonstrating their skill by punching through wooden boards or chopping a cinder-block in two. If they have this amazing ability, why don' they put it to good use? Instead of destroying perfectly good lumber they could be volunteering their time with a forest service trail crew and cut some wood that actually needs to be cut. Instead of smashing usable bricks they could help with the demolition of houses. Whats the deal?